1/ "上班" and "工作” both refer to "work", but “上班” could only be used as a verb, while "工作“ can be a verb and a noun as well.
When we want to express "he works at Apple Inc" ,we could say"他在苹果公司工作" or 他在苹果公司上班。
But when we say "he has a lot of work to do", we could only say "他有很多工作要做”。 We cannot say "他有很多上班要做。"
2/工作 also refers to "job", e.g. 找工作 look for a job
他的工作很有意思, his job is interesting.
3/工作单位 refers to the company, organization, institution which a person works with/for.